Picardie, 1963. A truck drops a group of illegal workers by a deserted stretch of canal in the dead of night near Poissons-les-Marais, desperate travellers in a pipeline, searching for a better life. Days later, one of them surfaces, stabbed to death. For Inspector Lucas Rocco, finding the victim's fellow travellers presents problems. Most Algerian immigrants are welcome, but trawling for any who aren't is a sensitive issue loaded with threats of civil unrest - something which terrifies his bosses in the Ministry. And when Rocco is ordered to stay away from one factory where he suspects illegals are employed, it goes against everything he knows. However, his difficulties have barely begun. One of the illegals is a young woman with a small child, on the run from her brutal husband, Samir Farek, an Algerian gangster who has threatened to kill them both. But Farek has another, bigger agenda: to take over as gang lord in Paris. With a history of disposing of anyone who crosses him - a fate he is happy to mete out even to a senior policeman who gets in his way - the potential consequences are frightening.
Caught between his instinct to protect a vulnerable woman and her child, of obeying bosses interested only in preserving the status quo, and facing the threat of a dangerous gangster who recognises no boundaries in the murderous pursuit of his plans, Rocco begins to wonder how long he can survive.